1. Field of the Invention
The present invention relates to a liquid crystal microcapsule for use in display devices, image/information-recording devices, image/information-recording media, spatial light modulators, a method for producing the same, and a liquid crystal display device using the same.
2. Description of the Related Art
Liquid crystals have been widely used as a material for displays. However, since liquid crystals are liquid in nature, in order to use them it is necessary to inject them into a cell formed between two substrates spaced at a certain distance. Liquid crystal systems are problematic in that it takes time to inject the liquid crystal and displayed images are easily distorted by changes in the distance between the two substrates when a force is applied to a cell, for example, by pushing or bending.
It is possible to overcome these problems by producing a liquid crystal microcapsule by encapsulating the liquid crystal with a film. This is advantageous in that the liquid crystal microcapsule, wherein the liquid crystal is protected by a film, is resistant to a pushing or bending force and eliminates the time-consuming process of injecting the liquid crystal as it is possible to form such a cell by coating the liquid crystal microcapsules on a substrate.
Usually, when a liquid crystal is used in a display device, control of the alignment of the liquid crystal is important for making the liquid crystal exhibit its inherent performance sufficiently. For example, Japanese Patent Application Laid-Open (JP-A) No. 2002-275471 discloses a method of improving display contrast by using a particular film material for a liquid crystal microcapsule, wherein a guest-host liquid crystal, i.e., a liquid crystal containing a dissolved dichroic colorant, is encapsulated, and thus controlling the alignment of the liquid crystal molecule in a direction perpendicular to the internal surface of the film.
FIGS. 4A to 4D show the alignment states of a liquid crystal in a liquid crystal microcapsule without alignment control and those of a liquid crystal in a liquid crystal microcapsule with alignment control. In FIGS. 4A to 4D, 11 represents a substrate; 12, an electrode; 13, a binder; and 14, a liquid crystal microcapsule. As shown in FIG. 4A, in the alignment-uncontrolled liquid crystal microcapsule, the liquid crystal becomes colored, orienting itself substantially in parallel with the substrate surface when no voltage is applied. When a nematic liquid crystal having a positive dielectric anisotropy is used as the liquid crystal and a voltage is applied between the two electrodes, the liquid crystal orients itself in the direction perpendicular to the substrate and becomes transparent as shown in FIG. 4B. At the interface with the film, the liquid crystal is oriented so as to curve along the interface, and thus there remain some areas in the liquid crystal where the liquid crystal is oriented perpendicular to the substrate surface, prohibiting sufficient color density.
On the other hand, when controlled so as to orient perpendicular to the film surface, the liquid crystal orients itself in a direction substantially perpendicular to the substrate surface and becomes transparent, as shown in FIG. 4C. When a nematic liquid crystal having a negative dielectric anisotropy is used as the liquid crystal and a voltage is applied between the two electrodes, as shown in FIG. 4D, the liquid crystal orients itself in a direction parallel to the substrate surface and become colored. In such a case, the liquid crystal, which is forcibly oriented in parallel with the substrate surface under application of a voltage, has a parallelism higher than that of FIG. 4A. Thus, the color density thereof is heightened and the display contrast is improved.
Accordingly, it is important to make the film internal surface more effective in inducing perpendicular alignment in order to improve the display performance of liquid crystal microcapsules.
As a method of making the film internal surface of liquid crystal microcapsules effective in inducing perpendicular alignment, a method of mixing a radical-polymerizable monomer having an alkyl group or a fluoroalkyl group with a liquid crystal, dispersing the mixture in water, and then forming a film by polymerization under heat is disclosed in JP-A No. 2002-275471 described above.
Because radical polymerization proceeds everywhere in the dispersion phase, it is necessary to devise a method of ensuring that the polymers move to the interface between the liquid crystal and the aqueous phase in order to form a uniform film of the generated polymers. Thus, it is necessary to control the surface tension of the polymer in a range between that of the liquid crystal and the external aqueous phase and to adjust the precipitation rate of the polymer to within a suitable range. However, the problem has existed that it has not been possible to form a film, or only possible to form a film having pores or inferior mechanical properties, because the conditions above have not always been satisfied depending on the kind of liquid crystal, and the combination thereof with the monomer, used. With such a film, it has only been possible to produce liquid crystal microcapsules inferior in the perpendicular aligning efficiency.
To avoid this problem, the same patent disclosed a method of strengthening the film by forming an additional film over liquid crystal microcapsules by adding a dispersion of the monomer to the dispersion of liquid crystal microcapsules having a film already formed, connecting monomers onto the film, and hardening the resulting film. However, the method was problematic in terms of the tedious procedures demanded for production.